Typical structures of trench MOSFETs having trenched floating gates as termination in prior arts are encountering technical problems. For example, in U.S. Pat. No. 6,462,376, a trench MOSFET was disclosed with n+ source region disposed in termination area comprising a plurality of trenched floating gates, as shown in FIG. 1A. In termination area, n+ source region 120 is disposed between two adjacent trenched floating gates 111. This kind of structure will cause heavily leakage current between drain region and source region as result of the channel is turned on in termination area due to floating P body regions 108 and the trenched floating gates 111 are not shorted together with source regions at drain/source reversed bias. An electrical current will flow from drain through channel region of the trenched floating gate and n+ source region 120 between two adjacent trenched floating gates 111 in termination area to the source region 120 in active area.
U.S. Pat. No. 7,511,339 disclosed another trench MOSFET structure without having n+ source region in termination area but with depth of trenched gates 110 shallower than depth of floating deep P body regions 130, as shown in FIG. 1B. However, from experimental results of the relationship between breakdown voltage (BV) and difference between depth of trenched floating gates (TFd) and depth of body region (Pd) in FIG. 2, we can see that, the breakdown voltage is significantly degraded when depth of trenched floating gate is shallower than deep body region, thus causing low breakdown voltage in termination area due to poor isolation between drain region and source region by the shallower trenched floating gates in termination area. The floating deep P body 130 regions are electrically connected together by charge depletion of the floating P body regions at drain-source reversed bias because the trenched floating gates is shallower than the floating P body. Therefore, an electrical current will directly flow from edge of the termination area to source region in active area without blocking by the shallower trenched floating gates in the termination.
Therefore, there is still a need in the art of the semiconductor device fabrication, particularly for trench MOSFET design and fabrication, to provide a novel cell structure, device configuration and fabrication process that would resolve these difficulties and design limitations. Specifically, it is desirable to maintain low breakdown voltage in termination area of a trench MOSFET.